Question: Simplify the following expression: $y = \dfrac{-2x^2- 17x- 35}{-2x - 7}$
Explanation: First use factoring by grouping to factor the expression in the numerator. This expression is in the form ${A}x^2 + {B}x + {C}$ First, find two values, $a$ and $b$ , so: $ \begin{eqnarray} {ab} &=& {A}{C} \\ {a} + {b} &=& {B} \end{eqnarray} $ In this case: $ \begin{eqnarray} {ab} &=& {(-2)}{(-35)} &=& 70 \\ {a} + {b} &=& &=& {-17} \end{eqnarray} $ In order to find ${a}$ and ${b}$ , list out the factors of $70$ and add them together. The factors that add up to ${-17}$ will be your ${a}$ and ${b}$ When ${a}$ is ${-7}$ and ${b}$ is ${-10}$ $ \begin{eqnarray} {ab} &=& ({-7})({-10}) &=& 70 \\ {a} + {b} &=& {-7} + {-10} &=& -17 \end{eqnarray} $ Next, rewrite the expression as $({A}x^2 + {a}x) + ({b}x + {C})$ $ ({-2}x^2 {-7}x) + ({-10}x {-35}) $ Factor out the common factors: $ x(-2x - 7) + 5(-2x - 7)$ Now factor out $(-2x - 7)$ $ (-2x - 7)(x + 5)$ The original expression can therefore be written: $ \dfrac{(-2x - 7)(x + 5)}{-2x - 7}$ We are dividing by $-2x - 7$ , so $-2x - 7 \neq 0$ Therefore, $x \neq -\frac{7}{2}$ This leaves us with $x + 5; x \neq -\frac{7}{2}$.